Capillary suction time (CST) tests with drilling and treatment fluids have been used to characterize formations for swelling tendencies since 1983. However, in most cases, it is not possible to establish comparisons between data sets acquired in different laboratories or at different times. This paper evaluates how certain factors impact CST and offers useable constraints to the method to establish relevant comparisons across different samples. The basic operation of CST tests is simple: slurries of formation materials are prepared in treatment fluids, and the time required for free liquid to travel a calibrated distance in standard porous paper is measured. Swelling formation materials consume free water from the slurry, decreasing the available free water and reducing the permeability of the filter cake, which increases the CST (Underdown and Conway 1992). Several factors can artificially alter the measured time, such as materials blocking the flow of fluid. Extensive experimentation to determine the most prevalent factors and minimize their significance led to a robust, sample-to-sample comparison methodology. The work presented in this paper considers changes in CST response based on the source of formation materials (outcrop samples, drilled cores, and cleaned drill cuttings), the composition [standard spiked samples and natural formation materials analytically characterized by X-ray diffraction (XRD) and cationic exchange capacity (CEC)], and sample preparation (grinding method, particle size, shear rate, shear time, slurry volume, and fluid effects) to determine how each factor impacts the measured CST. Ultimately, a method for obtaining consistent sample preparations and data interpretation for CST is presented. This technique is especially useful in low-permeability formations where other conventional methods, such as core flood tests, cannot be used. In particular, this technique has proven to be very useful when no core formation materials are available and only drilling cuttings can be obtained. To test the robustness of the methodology, shale formation materials from 30 unique wells with clay contents from 6 to 64% were characterized. Measured CST values for each material were used to assess the damage potential in relevant fluids and determine treatment recommendations. By implementing the proper testing constraints, the CST can be used as a reliable, quick, low-cost, easy-to-run, and quantifiable analytical technique for sample-to-sample comparisons. By controlling the parameters involved with sample preparation, CST can be used as a reliable means to accurately determine the formation material's sensitivity to a given fluid on a well-by-well basis.
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